Method for manufacturing a bump-attached wiring circuit board

ABSTRACT

An object of the present invention is to manufacture a bump-attached wiring circuit board with which stable bump connections are possible, and there is no need for bothersome operations such as plating pretreatments. A bump formation etching mask  7  is formed on the bump formation side  3   a  of a metal foil  3  having a thickness (t 1 +t 2 ) equal to the sum of the thickness t 1  of a wiring circuit  1  and the height t 2  of the bumps  2  to be formed on a wiring circuit  1 , the bumps  2  are formed by half-etching the metal foil  3  from the bump formation etching mask  7  side down to a depth corresponding to a predetermined bump height t 2 , and a metal thin film layer  10  composed of a different metal from the metal foil  3  is formed on the bump formation side of the metal foil  3 , thereby providing a bump-attached wiring circuit board with which stable bump connections are possible, and there is no need for bothersome operations such as plating pretreatments.

This is a Division of application Ser. No. 10/268,722 filed Oct. 11,2002, which in turn is a Division of application Ser. No. 09/895,210filed Jul. 2, 2001, now U.S. Pat. No. 6,518,510 issued on Feb. 11, 2003.The entire disclosure of the prior applications is hereby incorporatedby reference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a bump-attached wiring circuit boardhaving bumps of a uniform height, and to a method for manufacturing thisboard.

2. Description of the Related Art

It is common practice to make connections with microscopic bumps (suchas those with a diameter of 50 μm and a height of 30 μm) when connectinga wiring circuit board and electronic elements such as semiconductorelements or liquid crystal display elements, or when connecting layersin a multilayer wiring board.

FIGS. 3A to 3E illustrate a typical method for forming bumps of thissize.

First, as shown in FIG. 3A, a two-layer flexible board 33 is prepared byaffixing a copper foil 32 to a polyimide film 31, and the copper foil 32is patterned by photolithography to form a wiring circuit 34 (FIG. 3B).

Next, a cover coat layer 35 is formed by a conventional method on thewiring circuit 34 (FIG. 3C). For instance, a polyamic acid layer may beformed on the wiring circuit 34 and patterned by photolithography, and acover coat layer 35 formed by imidation. Alternatively, a resist ink maybe applied by printing.

Next, bump holes 36 are formed by irradiation with laser light in theregion of the polyimide film 31 corresponding to the wiring circuit 34(FIG. 3D), and then the cover coat layer 35 is covered with a protectivefilm (not shown) as needed, after which microscopic bumps are formed bygrowing metal bumps 37 over the wiring circuit 34 exposed at the bottomof the bump holes 36 (FIG. 3E).

However, when the bump holes 36 are made by irradiation with laserlight, there is variance in their open surface area due to variance inthe smear amount clinging to the bottom of the bump holes 36, and thisresults in the problem of considerable variance occurring in the heightof the metal bumps 37. This makes it difficult to achieve stable bumpconnection. It is particularly difficult to connect semiconductorelements all at once to a wiring circuit by ultrasonic connection. Aplating pretreatment is also essential in order to improve the adhesivestrength between the wiring circuit 34 and the metal bumps 37 formed onit.

SUMMARY OF THE INVENTION

The present invention attempts to solve the above-mentioned problemsencountered with prior art, and it is an object thereof to provide amethod for manufacturing a bump-attached wiring circuit board with whichstable bump connection is possible, and there is no need for bothersomeoperations such as plating pretreatment.

The inventors arrived at the present invention upon discovering thatbumps of a uniform height can be produced, without performing bothersomeoperations such as plating pretreatment, by half-etching a metal foil,whose thickness is equal to the sum of the thickness of the wiringcircuit layer and a thickness corresponding to the metal bump height,down to a depth corresponding to the metal bump height, and that if ametal thin film layer composed of a different metal from the metal foilis formed on the bump formation side of the metal foil, then adhesionwill be improved between the bump formation side of the metal foil andthe insulating layer above, the chemical resistance of the wiringcircuit board will be improved, separation between the metal foil on thebump side and the insulating layer above will be prevented, and stablebump connection can be achieved.

Specifically, the present invention is a bump-attached wiring circuitboard in which a cover coat layer is formed on one side of a wiringcircuit, an insulating layer is formed on the other side, and bumps thatare electrically connected to the wiring circuit are formed protrudingfrom the insulating layer, wherein the wiring circuit and the bumps areintegrally formed from a single metal foil, and a metal thin film layercomposed of a different metal from said metal foil is provided betweenthe insulating layer and the side of the wiring circuit on which thebumps are formed.

Here, when the insulating layer is a polyimide film produced by theimidation of a polyimide precursor layer, it is preferable if the metalthin film layer exhibits higher adhesive force with respect to thepolyimide precursor layer than to the metal foil. Favorable examples ofcombinations of the metal foil and metal thin film here include a metalfoil that is copper foil, combined with a metal thin film of nickel,zinc, tin, or a nickel-cobalt alloy. In this bump-attached wiringcircuit board, it is preferable for the cover coat layer to have aconnection opening for allowing access to the wiring circuit from thecover coat side.

The present invention is also a method for manufacturing a bump-attachedwiring circuit board in which bumps are formed on a wiring circuit,comprising the steps of:

(a) laminating a protective film over the bump formation side of a metalfoil having a thickness equal to the sum of the thickness of the wiringcircuit and the height of the bumps to be formed on the wiring circuit,and forming a wiring circuit formation etching mask on the wiringcircuit formation side of the metal foil;

(b) forming a wiring circuit in a predetermined thickness byhalf-etching the metal foil from the side with the wiring circuitformation etching mask;

(c) removing the wiring circuit formation etching mask, and thenproviding a cover coat layer to the wiring circuit;

(d) removing the protective film provided to the bump formation side ofthe metal foil, and then forming a bump formation etching mask on thisbump formation side;

(e) forming bumps of a predetermined height by half-etching the metalfoil from the bump formation etching mask side;

(f) removing the bump formation etching mask, and then forming a metalthin film layer composed of a different metal from that of the metalfoil;

(g) forming a polyimide precursor layer over the metal thin film layerso as to bury the bumps; and

(h) etching back the polyimide precursor layer and forming an insulatinglayer in a predetermined thickness by imidation.

With this manufacturing method, the wiring circuit is formed before thebumps are formed.

Further, the present invention is a method for manufacturing abump-attached wiring circuit board in which bumps are formed on a wiringcircuit, comprising the steps of:

(aa) laminating a protective film over the wiring circuit formation sideof a metal foil having a thickness equal to the sum of the thickness ofthe wiring circuit and the height of the bumps to be formed on thewiring circuit, and forming a bump formation etching mask on the bumpformation side of the metal foil;

(bb) forming bumps of a predetermined height by half-etching the metalfoil from the side with the bump formation etching mask;

(cc) removing the bump formation etching mask, and then forming a metalthin film layer composed of a different metal from that of the metalfoil;

(dd) forming a polyimide precursor layer over the metal thin film layerso as to bury the bumps;

(ee) etching the polyimide precursor layer and forming an insulatinglayer in a predetermined thickness by imidation;

(ff) removing the protective film provided to the wiring circuitformation side of the metal foil, and then forming a wiring circuitformation etching mask on this wiring circuit formation side;

(gg) forming a wiring circuit in a predetermined thickness byhalf-etching the metal foil from the wiring circuit formation etchingmask side; and

(hh) removing the wiring circuit formation etching mask, and thenproviding a cover coat layer to the wiring circuit.

With this manufacturing method, the bumps are formed before the wiringcircuit is formed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1H are diagrams illustrating the steps in the method of thepresent invention for manufacturing a bump-attached wiring circuitboard;

FIGS. 2A to 2I are diagrams illustrating the steps in the method of thepresent invention for manufacturing a bump-attached wiring circuitboard; and

FIGS. 3A to 3E are diagrams illustrating the steps in a conventionalmethod for manufacturing a bump-attached wiring circuit board.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The various steps in the manufacture of the bump-attached wiring circuitboard of the present invention will now be described in detail throughreference to the drawings.

First, a method for manufacturing a bump-attached wiring circuit boardin which bumps are formed on a wiring circuit, and in which the wiringcircuit is formed before the bumps are formed (steps (a) to (h)), willbe described step by step through reference to FIGS. 1A to 1H.

Step (a)

First, a protective film 4 is laminated on the bump formation side 3 aof a metal foil 3 having a thickness equal to the sum of the thicknesst1 of a wiring circuit 1 (see dotted line in figure) and the height t2of the bumps 2 (see dotted line in figure) to be formed on the wiringcircuit 1, and a wiring circuit etching mask 5 is formed on the wiringcircuit formation side 3 b of the metal foil 3 (FIG. 1A).

The optimal values are selected for the thickness t1 of the wiringcircuit and the height t2 of the bumps 2 according to the intended useof the wiring circuit board. For example, if the wiring circuit board isto be used as a mounting board for semiconductor elements, then thethickness t1 of the wiring circuit can be set at 20 μm, the height t2 ofthe bumps 2 at 30 μm, and the diameter of the bumps 2 at 50 μm.

Any material used in the conductor layer of a wiring circuit board canbe used as the metal foil 3, but the use of a copper foil is preferred.

The wiring circuit etching mask 5 can be formed by the screen printingof a resist ink on the wiring circuit formation side 3 b of the metalfoil 3. Alternatively, a photosensitive resin layer or a dry film can beprovided and patterned by exposure and developing according toconventional methods.

Step (b)

Next, the metal foil 3 is half-etched from the wiring circuit etchingmask 5 side to form the wiring circuit 1 in the conventional thicknesst1 (FIG. 1B).

The half-etching conditions (such as temperature and etching liquidcomposition) can be suitably selected according to the material of themetal foil 3, the thickness to be etched, and so forth.

Step (c)

The wiring circuit etching mask 5 is then removed by a conventionalmethod, after which the cover coat layer 6 is provided to the wiringcircuit 1 (FIG. 1C).

The cover coat layer 6 can be formed by the screen printing of a coatingmaterial for the cover coat layer. Alternatively, a photosensitive resinlayer or a dry film can be provided and patterned by exposure anddeveloping according to conventional methods. Also, a layer composed ofa polyimide precursor such as polyamic acid can be provided, patterned,and imidated by conventional methods to form the cover coat layer 6.

In providing the cover coat layer 6, it is preferable to provide aconnection opening 11 that will allow access to the wiring circuit 1from the cover coat layer 6 side.

Step (d)

The protective film 4 provided to the bump formation side 3 a of themetal foil 3 is removed by a conventional method, after which a bumpformation etching mask 7 is formed on the bump formation side 3 a (FIG.1D).

The bump formation etching mask 7 can be formed by the screen printingof a resist ink on the bump formation side 3 a. Alternatively, aphotosensitive resin layer or a dry film can be provided and patternedby exposure and developing according to conventional methods.

Step (e)

Bumps 2 of a predetermined height t2 are formed by half-etching themetal foil 3 from the bump formation etching mask 7 side (FIG. 1E).

The half-etching conditions (such as temperature and etching liquidcomposition) can be suitably selected according to the material of themetal foil 3, the thickness to be etched, and so forth.

Prior to this half-etching, the cover coat layer 6 may be covered with aprotective film (not shown).

Step (f)

The bump formation etching mask 7 is removed by a conventional method,after which a metal thin film layer 10 composed of a different metalfrom the metal foil 3 is formed (FIG. 1F).

It is preferable for the metal thin film layer 10 to be formed from ametal material that exhibits high adhesive force with respect to apolyimide precursor layer 8 (discussed below). This will afford betteradhesion between the polyimide precursor layer 8 and the metal foil 3,and therefore in subsequent chemical treatment (such as the etch-back ofthe polyimide precursor layer 8 in step (g)), separation can beprevented between the metal foil 3 and the polyimide precursor layer 8or an insulating layer 9 (see step (h)) produced by the imidation ofthis polyimide precursor layer 8.

Favorable examples of this metal thin film layer 10 when the metal foil3 is an ordinary copper foil include metal thin films of nickel, zinc,tin, and a nickel-cobalt alloy. These thin films can be formed byelectroless plating, electrolytic plating, vacuum vapor deposition, oranother such method.

If the metal thin film layer 10 is too thin, adhesion cannot besufficiently improved between the insulating layer 9 and the wiringcircuit 1, but if it is too thick, the obtained effect will notcorrespond to the increased thickness, so a preferable range is 0.01 to4 μm. In particular, when the metal thin film layer 10 is a thin film ofzinc or tin, the preferred thickness of this layer is 0.1 to 0.5 μm;when the metal thin film layer 10 is a thin film of a nickel-cobaltalloy, the preferred thickness of this layer is 0.1 to 4 μm; and when itis a thin film of nickel, the preferred thickness of this layer is 0.01to 1 μm.

In the removal of the bump formation etching mask 7, if the cover coatlayer 6 is covered with a protective film, the protective film may beremoved at the same time.

Step (g)

The polyimide precursor layer 8 is formed over the metal thin film layer10 so as to bury the bumps (FIG. 1G).

The polyimide precursor layer 8 can be formed by a conventional filmformation process from polyamic acid or the like. The imidationconditions can also be determined according to the type of polyimideprecursor and so forth.

Step (h)

The polyimide precursor layer 8 is etched back and imidated to form theinsulating layer 9 in a predetermined thickness t3. This yields thebump-attached wiring circuit board shown in FIG. 1H.

Next, a method for manufacturing a bump-attached wiring circuit board inwhich bumps are formed on a wiring circuit, and in which the bumps areformed before the wiring circuit is formed (steps (aa) to (hh)), will bedescribed step by step through reference to FIGS. 2A to 2I. Thestructural elements in FIGS. 2A to 2I correspond to the structuralelements numbered the same in FIGS. 1A to 1H.

Step (aa)

First, a protective film 4 is laminated on the wiring circuit formationside 3 b of a metal foil 3 having a thickness equal to the sum of thethickness t1 of a wiring circuit 1 (see dotted line in figure) and theheight t2 of the bumps 2 (see dotted line in figure) to be formed on thewiring circuit 1, and a bump formation etching mask 7 is formed on thebump formation side 3 a of the metal foil 3 (FIG. 2A).

Step (bb)

The metal foil 3 is half-etched from the bump formation etching mask 7side to form the bumps 2 of a predetermined height t2 (FIG. 2B).

Step (cc)

The bump formation etching mask 7 is removed by a conventional method,after which a metal thin film layer 10 composed of a different metalfrom the metal foil 3 is formed (FIG. 2C).

Step (dd)

The polyimide precursor layer 8 is formed over the metal thin film layer10 so as to bury the bumps 2 (FIG. 2D).

Step (ee)

The polyimide precursor layer 8 is etched back and imidated to form theinsulating layer 9 in a predetermined thickness t3 (FIG. 2E).

Step (ff)

The protective film 4 provided to the wiring circuit formation side 3 bof the metal foil 3 is removed by a predetermined method, after which awiring circuit etching mask 5 is formed on the wiring circuit formationside 3 b (FIG. 2F).

Step (gg)

The metal foil 3 is half-etched from the wiring circuit etching mask 5side to form the wiring circuit 1 in the predetermined thickness t1(FIG. 2G).

Prior to this half-etching, the bumps 2 may be covered with a protectivefilm (not shown).

Step (hh)

The wiring circuit etching mask 5 is removed by a predetermined method(FIG. 2H), after which the cover coat layer 6 is provided to the wiringcircuit 1. This yields the bump-attached wiring circuit board shown inFIG. 2I. It is preferable here to provide a connection opening 11 thatwill allow access to the wiring circuit 1 from the cover coat layer 6side.

In the removal of the wiring circuit etching mask 5, if the bump 2 iscovered with a protective film, the protective film may be removed atthe same time.

As shown in FIG. 1H and FIG. 2I, the bump-attached wiring circuit boardobtained by the above manufacturing method of the present invention issuch that the cover coat layer 6 is formed on one side of the wiringcircuit 1, the insulating layer 9 is formed on the other side, and thebumps 2, which are electrically connected to the wiring circuit 1, areformed protruding from the insulating layer 9, wherein the wiringcircuit 1 and the bumps 2 are integrally formed from a single metalfoil, and the metal thin film layer 10 composed of a different metalfrom said metal foil is provided between the insulating layer 9 and theside of the wiring circuit 1 on which the bumps 2 are formed. When theinsulating layer 9 is a polyimide film produced by the imidation of apolyimide precursor layer, this metal thin film layer 10 is preferablyformed from a material that exhibits higher adhesive force with respectto the polyimide precursor layer than to the metal foil. For example,when the metal foil is a copper foil, the metal thin film layer 10 isformed from a thin film of nickel, zinc, tin, or a nickel-cobalt alloy.This affords better adhesion between the polyimide precursor layer andthe metal foil, and therefore, in a chemical treatment (such as theetch-back of the polyimide precursor layer), separation can be preventedbetween the metal foil and the polyimide precursor layer or aninsulating layer 9 produced by the imidation of this polyimide precursorlayer.

Also, since the cover coat layer 6 of this bump-attached wiring circuitboard has a connection opening 11 that allows access to the wiringcircuit 1 from the cover coat layer 6 side, the board can be accessedfrom both sides, and this helps in raising the mounting density ofelectronic elements.

The present invention provides a bump-attached wiring circuit board withwhich the bump strength is stable, stable bump connections are possible,and there is no need for bothersome operations such as platingpretreatments. In particular, bump connections on an integrated circuitcan be stably produced all at once ultrasonically.

The entire disclosure of the specification, claims, summary and drawingsin Japanese Patent Application No. 2000-210482 filed on Jul. 11, 2000 ishereby incorporated by reference.

1. A method for manufacturing a bump-attached wiring circuit board inwhich bumps are formed on a wiring circuit, comprising the steps of:(aa) laminating a protective film over a wiring circuit formation sideof a metal foil having a thickness equal to the sum of the thickness ofthe wiring circuit and the height of the bumps to be formed on thewiring circuit, and forming a bump formation etching mask on a bumpformation side of the metal foil; (bb) forming bumps of a predeterminedheight by half-etching the metal foil from the side with the bumpformation etching mask; (cc) removing the bump formation etching mask,and then forming a metal thin film layer over the bumps, the metal thinfilm layer being composed of a different metal from that of the metalfoil; (dd) forming a polyimide precursor layer over the metal thin filmlayer so as to bury the bumps; (ee) etching the polyimide precursorlayer and forming an insulating layer above the metal thin film layer ina predetermined thickness by imidation; (ff) removing the protectivefilm provided to the wiring circuit formation side of the metal foil,and then forming a wiring circuit formation etching mask on this wiringcircuit formation side; (gg) forming a wiring circuit in a predeterminedthickness by half-etching the metal foil from the wiring circuitformation etching mask side; and (hh) removing the wiring circuitformation etching mask, and then providing a cover coat layer to thewiring circuit.
 2. The manufacturing method according to claim 1,wherein the metal thin film layer formed in step (cc) exhibits higheradhesive force with respect to the polyimide precursor layer than to themetal foil.
 3. The manufacturing method according to claim 1, whereinthe metal foil is a copper foil, and the metal thin film layer is a thinfilm of nickel, zinc, tin, or a nickel-cobalt alloy.
 4. Themanufacturing method according to claim 3, wherein the thickness of themetal thin film layer is 0.01 to 4 μm.
 5. The manufacturing methodaccording to claim 3, wherein, when the metal thin film layer is a thinfilm of zinc or tin, the thickness of the metal thin film layer is 0.1to 0.5 μm.
 6. The manufacturing method according to claim 3, wherein,when the metal thin film layer is a thin film of a nickel-cobalt alloy,the thickness of the metal thin film layer is 0.1 to 4 μm.
 7. Themanufacturing method according to claim 3, wherein, when the metal thinfilm layer is a thin film of nickel, the thickness of the metal thinfilm layer is 0.01 to 1 μm.
 8. The manufacturing method according toclaim 2, wherein the metal foil is a copper foil, and the metal thinfilm layer is a thin film of nickel, zinc, tin, or a nickel-cobaltalloy.